1. Field of the Invention
This invention relates to a nematic liquid crystal compound of four ring systems useful as an electro-optical display material.
2. Prior Art
Typical liquid crystal display cells include a field-effect mode cell proposed by M. Schadt et al. (cf. Appld. Phys. Letters, 18, 127-128 (1971)), a dynamic scattering mode cell proposed by G.H. Heilmeier et al. (cf. Proc. of the I.E.E.E., 56, 1162-1171 (1968)) and a guest-host mode cell proposed by G.H. Heilmeier et al. (cf. Appld. Phy. Letters, 13, 91 (1968)) or D.L. White et al. (J. of Appld. Phys., 45, 4718 (1974)).
Among various requirements for these liquid crystal display cells, it is a commonly important one to have a nematic phase over a wide range of temperature involving room temperature. Many of commercially feasible liquid crystal materials available today are usually prepared by mixing a few or more components made of a compound having a nematic phase at or around room temperature and another one having a nematic phase within a temperature range higher than room temperature. Most of these mixed liquid crystals commercially used today are required to have a nematic phase over the full temperature range of -50.degree. to +65.degree. C. In order satisfy these requirements, compounds having a crystalline to nematic (C-N) transition temperature of approximately 100.degree. C. and a nematic to isotropic (N-I) transition temperature of approximately 200.degree. C., for example, 4,4"-substituted terphenyl, 4,4'-substituted biphenylcyclohexane or phenyl 4,4'-substituted benzoyloxybenzoate are frequently employed as the compound having a nematic phase within a temperature range higher than room temperature. Recently there are required liquid crystal materials having a nematic phase within a further higher temperature range. This requirement is satisfied by providing a compound having an N-I transition temperature as high as approximately 300.degree. C.
Prior known compounds similar to the compound of this invention are disclosed in Japanese Patent Application (OPI) No. 92228/85 (the term "OPI" refers to unexamined published Japanese patent application), and U.S. Pat. No. 4,472,293.
However these compounds usually have a disadvantage that each has a smectic phase and a high smectic to nematic (S-N) transition temperature which corresponds to the lower limit of the nematic phase temperature range. When a compound having a high S-N transition temperature is added to mixed nematic liquid crystals, which are widely used at present as a liquid-crystal host, to thereby elevate the N-I transition temperature of the mixed liquid crystals, the resulting mixture is undesirable in having a smectic phase within a low temperature range.